This invention relates to a counterbalance apparatus for use in an ion implanter.
As will be familiar to those skilled in the art, in a typical ion implanter a relatively small cross-section beam of dopant ions is scanned relate to a silicon wafer. Traditionally, a batch of wafers was mechanically scanned in two directions relative to a fixed direction ion beam.
With the advent of larger wafers, up to 300 mm in diameter, processing of a single wafer at a time has advantages in terms of cost and reduced wastage etc. Accordingly, it is now desirable to scan an ion beam relative to a silicon wafer by mechanically scanning the wafer in a first direction and electrostatically or electromagnetically scanning or fanning the ion beam in a second direction.
There are a number of different configurations of single wafer processing machines. One example is described in WO99/13488 and other configurations are described in U.S. Pat. Nos. 5,003,183 and 5,229,615. In WO99/13488, the wafer is mounted upon a substrate holder in a process chamber of an implantation device. Attached to, or integral with, the substrate holder is an arm which extends through an aperture in the wall of the vacuum chamber. Mechanical scanning is effected by a scanning mechanism located outside the process chamber. The scanning mechanism is connected with the arm of the substrate holder and allows movement of the arm and hence the substrate holder relative to the process chamber.
Typically, the arm is substantially horizontal and is moved up and down in a reciprocating motion by a servo driven linear motor. Since the arm and substrate holder have significant combined mass, the effect of gravity on this mass makes it difficult for the servo and motor to provide precise velocity control throughout the upward and downward strokes.
It is an object of the present invention to address this problem and to provide a scanning system with a counterbalance device to achieve a substantially constant force system.
Accordingly, the present invention provides an ion implanter comprising: an ion beam generator to generate a beam of ions to be implanted; a process chamber into which the ion beam is directed, the process chamber having a wall defining an aperture; a scanning device movable in linear reciprocating motion along a first axis relative to said process chamber; a wafer support structure having a longitudinal axis and being mounted within said process chamber with said axis substantially horizontal, the wafer support structure having an end protruding out of said process chamber through said aperture and being secured to said scanning device; at least one piston attached to said end of said wafer support structure, said at least one piston having first and second opposed faces; at least one cylinder slidably receiving said at least one piston for reciprocating movement along a second axis parallel to said first axis, said at least one cylinder having a first end which is open to allow atmospheric pressure to act on said first face of said piston, and said at least one cylinder defining a chamber enclosed by said second face of said piston, said chamber, in use, being evacuated.
Preferably the ion implanter further comprises a rotor mounted on said wall of said process chamber for rotation relative to said process chamber, said rotor having an aperture through which said end of said wafer support surface passes; said scanning device and said at least one piston being mounted on said rotor for linear reciprocating movement relative thereto.
Preferably the first face of the piston has an area selected such that when the second axis is substantially vertical, atmospheric pressure acts upon the first face of the piston to create an upward force substantially equal to a downward force acting on the wafer support surface and the scanning means due to gravity.
Conveniently the rotor is rotatable relative to the process chamber, in use, on a fluid bearing layer. The scanning means may also be movable relative to the rotor, in use, on a fluid bearing layer. In either case the fluid of said fluid bearing layer may be compressed air.
The ion implanter preferably further comprises a linear motor for moving said scanning means. A servo control system may be provided for controlling the linear motor. Preferably, the motor is able to provide that the scanning means is reciprocatable relative to the process chamber at a velocity of about 40 cm/s and with a turnaround time of about 100 ms.
The chamber of the at least one cylinder is preferably evacuated, in use, to about 10 Torr.